HIV counseling and testing is one central component of HIV prevention and treatment efforts in the US. Our own prior research and that of others have found that significant proportions of persons at high risk for HIV have never been tested or have not had a recent HIV test, despite the increasing benefits of early HIV testing to access early treatment. There is a critical need for new research aimed at understanding and removing the barriers preventing at-risk populations from seeking an initial and subsequent HIV tests. Philadelphia Health Management Corporation (PHMC) is proposing a study to develop targeted strategies to increase HIV testing among high-risk populations and subgroups. The proposed study will: 1) conduct a broad cross-sectional community-based study of factors related to HIV testing among three risk groups: men who have sex with men (MSM), injection drug users (IDU), and persons who engage in high risk heterosexual sexual behavior; 2) expand the current conceptualization of factors related to HIV testing by focusing on three domains: sociodemographic, psychosocial, and community and structural determinants of HIV testing and how the relationship of these three factors impacts HIV testing; 3) examine HIV testing as a continuum of behavior rather than a dichotomous measure; and 4) Investigate the effect of emerging testing technologies, treatments, and reporting requirements on HIV testing behaviors. Data will be collected through a community survey of 1,500 persons at risk for HIV infection because of their drug use and/or sexual behaviors and a targeted sample of 500 MSM. A two stage random sampling design will be used to select and interview an unbiased sample of people who have never been tested for HIV, persons tested who did not return for test results, those who have not been tested recently, and persons who have recently been tested. Persons will be selected across diverse city neighborhoods with already high or increasing rates of AIDS cases. Data will be analyzed using bivariate and multivariate techniques (CHAID Segmentation Modeling and structural equation modeling) to specify the determinants of HIV testing behavior and variations across HIV risk and population subgroups. Geographic Information Systems (GIS) software will be used to graphically show the location of non-testers and structural and community-level variables, including HIV prevention programs and other health care resources. The results of the proposed study will be important because they will enhance our understanding of factors related to HIV testing, provide needed information to health educators and health care providers, and inform the development policy recommendations.